Liquid crystals are traditionally classified as thermotropic, lyotropic, or polymeric based on the stimulus that governs the organization and order of the molecular system. Subjecting liquid crystals to a stimulus, for example by heating can decrease the ability of the material system to organize resulting in transitions from or into a number of well-known mesophases including nematic, cholesteric, numerous variations of smectic, in addition to more complicated phases such as the blue phase. To date, there has been no report of a liquid crystal mixture wherein light irradiation serves to increase the order of the system allowing for order-increasing phase transitions.
Recently, there has been growing interest in photoresponsive liquid crystals in which light can be used to remotely control physical and/or optical properties such as the color of the optical reflection band, surface tension, and mechanical properties of polymeric liquid crystalline systems. In addition to the unique capability of remote control, light can readily be spatially patterned with holography or shadow-masking, potentially useful as a photodisplay.
In the past, azobenzene photochromic compounds have predominantly been employed as the photochromic moiety of choice in the formulation of photoresponsive liquid crystal material systems because of their rod-like shape that can allow for synthesis of azobenzene-containing liquid crystal molecules as well as excellent compatibility with conventional liquid crystalline systems in guest-host mixtures.
The photosensivity of photochromic materials has been long employed as a smart material technology, in which the material autonomously cues a desired functional response, most plainly evident in variable transmission (photochromic) eyewear that darken in the presence of sufficient sunlight. Over the years, photosensitive liquid crystals systems have been realized as guest-host mixtures of liquid crystals with non-liquid crystalline photochromics, or by the synthesis and employment of photochromic liquid crystal molecules. A few reports detail employment of spiropyran, spriooxazine, and napthopryan photochromic moieties in opthamalic applications, synthesis of mesogenic molecular targets based on these photochromics, or using photochromism in a guest-host mixture. In these systems, the photochromics are a guest in the liquid crystal host and do not affect the physical or optical properties of the host liquid crystals.
In addition to simple changes in the physical properties, some photoresponsive liquid crystal materials and mixtures can undergo transitions from a higher ordered phase to a lower ordered phase if light significantly affects the order parameter of the system within a mesophase. Light exposure of previously examined photoresponsive liquid crystalline materials (guest-host or mesogenic) from their ground state results in a decrease in order, with sufficient irradiation resulting in what has been referred to as a photoinduced isothermal phase transition. Under very specialized cases, an artificial increase in order with light exposure can be achieved in some systems. In the first instance, investigators have observed increases in order in an azobenzene doped Smectic A (SmA) phase with light exposure. (A. Chanishvili, G. Chilaya, G. Petriashvili, D. Sikharulidze, Mol. Cryst. Liq. Cryst. 409, 209 (2004)). The increase in order was found to be caused by photoinduced phase separation of cis-azobenzene from the Sm layers. In other words, the system exhibited a micro phase separation of the guest dye from the host and as such was not thermodynamically stable system.
This type of unstable phase separation has also been employed to suppress a “reentrant” nematic phase to yield a higher-ordered SmA phase. The reentrant nematic phase is found to precede the appearance of the higher ordered SmA phase. Others have taken the approach of subjecting photoresponsive liquid crystals to “pre-exposure” to one color of light to drive the system to the disordered isotropic state (typically metastable), with subsequent illumination by longer wavelength light resulting in a restoration of a higher ordered, cholesteric phase. As in the previous cases, these system are thermodynamically unstable before the exposure. In short, the apparent increase in order is only due to a thermodynamically unstable situation.
To our knowledge, there are no prior reports of photoresponsive liquid crystalline systems in which light directly increases the order parameter of a thermodynamically stable liquid crystal material system without inducing a phase separation. In addition there are no reports of a light induced phase transition from isotropic to a liquid crystalline phase or transition within the different meso phases wherein the phase after exposure has a higher degree of order. Here, we present the development of a novel class of guest-host liquid crystal materials that uniquely exhibit an increase in order parameter upon irradiation. Depending on the state of the system, the materials may also be capable of yielding order-increasing phase transitions.